S60 Poor Starting Terminal Interruption

Sounds like a power supply issue. Similar problem with my old BMW 3 series. Have you checked the battery voltage, or noticed any other electrical weirdness? Voltage drop can cause gremlins.

Voltage drop? Where do I even start to check? What else is electrical weirdness?

Electrical weirdness is lights flickering, or infotainment acting strangely. Check voltage drop with a multimeter at battery and starter motor while cranking. Workshop might be best if you're not comfortable with electrical diagnostics. Could also be charging system or ignition switch.

Took it to a mechanic, you were spot on – battery was heavily discharged and defective. Replaced it, starting issue is gone. Cost 290€.